In a conventional package-on-package (POP) process, a top package, in which a first device die is bonded, is further bonded to a bottom package. The bottom package may also be bonded with a second device die. As a result, the second device die may be on the same side of the bottom package as the solder balls that are used to bond the bottom package to the top package.
Before the bonding of the top package to the bottom package, a molding compound is applied on the bottom package, with the molding compound covering the second device die and the solder balls. Since solder balls are buried in the molding compound, a laser ablation or drilling is performed to form holes in the molding compound, so that the solder balls are exposed. The top package and the bottom package may then be bonded through the solder balls in the bottom package.